In many HIV-1 -infected patients on highly active antiretroviral therapy (HAART), plasma virus levels fall to below the limit of detection of ultrasensitive clinical assays (50 copies of HIV-1 RNA/ml). However, HIV-1 persists in a latent form in resting CD4+ T cells and possibly in other cellular reservoirs. In addition, low levels of free virus can be found in the plasma with special methods. This low-level residual viremia (RV) may reflect release of virus from latently infected cells that become activated or release of virus from other stable cellular reservoirs. Alternatively, RV may reflect ongoing cycles of viral replication that continue despite HAART. Analysis of RV is technically difficult but important because it provides a "window" into the state of virologic suppression, revealing the significance of different proposed mechanisms of viral persistence. In future studies, we will address three critical questions raised by our initial analysis of RV. First, we will determine whether ongoing replication contributes to RV in patients on HAART. A major unsolved question in the field of HIV-1 treatment is whether HAART can completely stop viral replication. Our initial analysis of RV demonstrated that, in the optimal situation, there was an apparent arrest of viral evolution, consistent with the hypothesis that RV reflects release of virus from stable reservoirs rather than continuing cycles of replication. To confirm this hypothesis, we will analyze samples from two intensification studies in which patients on effective HAART will have potent inhibitors of HIV-1 protease or integrase added to their regimens. Careful analysis of the qualitative and quantitative effects of intensification on RV will indicate whether ongoing replication occurs in such patients. Second, we will determine the clinical significance of the predominant plasma virus clones that constitute most of the RV in many patients on HAART. The most striking finding to emerge from our initial studies was that in half of the patients studied, the bulk of the RV was comprised of a small number of predominant plasma clones (PPC) that were released into the circulation over prolonged periods without evolution by a cell type not well represented in the peripheral blood. These results raise the possibility that there is a second major reservoir that contributes to viral persistence in patients on HAART. In the proposed studies, we will determine the clinical significance of this unexpected observation. Third, we will examine critical aspects of env and vpr gene function for these PPCs in an effort to understand more about the cellular source of the PPC. Lay language description: By looking at the very low amount of virus in the blood of patients on combination therapy who have "undetectable" viral loads, we will determine whether combination therapy can completely stop the virus from replicating and look for additional reservoirs in which the virus can hide.